1. Field of the Invention
The present invention is generally concerned with multiple access radiocommunication systems.
2. Description of the Prior Art
Various multiple access techniques exist, including the code-division multiple access (CDMA) technique and the time-division multiple access (TDMA) technique. Generally speaking, the CDMA technique has been in vogue in recent years and is now embodied in a number of standards for terrestrial communications and for satellite systems.
There are two basic CDMA techniques. The first uses pseudo-random sequences to spread the spectrum at the transmitter and to despread it at the receiver. This technique is referred to hereinafter as the PN-CDMA (pseudo-noise CDMA) technique. The second technique uses orthogonal sequences such as Walsh-Hadamord sequences. This technique is referred to hereinafter as the OCDMA (orthogonal CDMA) technique.
The theory of multiple access techniques like the TDMA, OCDMA and PN-CDMA techniques referred to above will now be briefly described.
Consider a point-multipoint radiocommunication system in which a central station serves N users.
The TDMA technique time-division multiplexes the signals of the various users to form a time-division multiplex also referred to as a frame. The uplink frame is divided into N time slots and each time slot is allocated to one user. In other words, the signals emanating from the various users are separated in time so that they con be detected by the central station with no mutual interference. If W Hz is the bandwidth needed to transmit a bit rate of D bauds using the modulation technique employed, a TDMA system with N users each transmitting a bit rate of D bauds requires a bandwidth of N.W Hz.
Generally speaking, the CDMA technique is based on spectrum spreading using direct sequences and originated in military communication systems. The two attributes of spectrum spreading are discretion (the signal is buried in the noise) and robustness in the face of narrowband jamming.
The OCDMA technique uses sequences that are totally orthogonal to each other. Accordingly there is no mutual interference between the spread signals emanating from the various users. If the bandwidth available on the uplink channel to the central station is N times the bandwidth needed for each individual user, then the OCDMA technique caters for exactly N users, because the number of orthogonal sequences of length N is indeed N. This indicates that the capacity of the OCDMA technique is exactly the same as that of the TDMA technique.
In the PN-CDMA technique, the spreading sequences are pseudo-random sequences and are therefore not orthogonal. Consequently, there is interference as soon as there are two active users. If all the signals are of the same power, the interference from one user to another has a power of 1/N, if the power of the wanted signal is normalized at 1. If p users are active, each user receives interference at a level of (pxe2x88x921)/N from the other pxe2x88x921 users. Assuming that N users are active, the ratio of the wanted signal to the total interference is then equal to (Nxe2x88x921)/N (i.e. almost equal to 1), which indicates that it is not possible to have N users in a PN-CDMA system in which the spreading factor is N. The number of users that can be served is directly related to the deterioration that can be accepted. For example, if interference is to be limited to 30% of the power of the wanted signal, the number of users must be limited to 0.3N, which is more than three times less than the capacity of TDMA and OCDMA systems.
The foregoing discussion shows that the smallest capacity is that of the PN-CDMA technique. It might even be thought that this is not a natural multiple access technique, because there is interference as soon as there are two active users, whereas with the other techniques there is strictly no interference up to a number N of users. On the other hand, the conventional TDMA and OCDMA techniques cannot add even one additional user once the maximum number has been reached.
One object of the present invention is to provide a variant of the above multiple access technique offering greater capacity than the conventional technique.
A more particular object of the present invention is to provide a transmission method for use in multiple access radiocommunication systems with orthogonal transmission resources which cater for a number M of users greater than the number N of transmission resources of the system and in which there is no deterioration of the signal to noise ratio for a number of users less than or equal to N and the deterioration of the signal to noise ratio is minimal if the number of users becomes greater than N.
The present invention therefore consists in a transmission method in a multiple access radiocommunication system with orthogonal transmission resources, in which method, for a number of users greater than the number of transmission resources of the system:
at least one of said transmission resources is shared at a given time by at least two users,
the users sharing a transmission resource change with time so as to divide the deterioration of the signal to noise ratio resulting from such sharing as evenly as possible between the various users, and
the deterioration of the signal to noise ratio is further reduced by differentiating the signals corresponding to users sharing the same resource at a given time in terms of their transmit level.
The present invention equally consists in a transmitter and a receiver for implementing the above method.